The BHA is subjected to precession when it undergoes a proper rotating motion notably due to flexion. This proper motion is influenced by drilling parameters (weight on bit, rotating speed, etc.) and geometric parameters (dimensions of the BHA constituents, bit diameter, wellbore trajectory, formation type, etc.). It is considered to be a serious dysfunctioning since it prevents correct progress of the drilling operation and can even cause considerable material damage, notably to the BHA elements.
The dysfunctioning known as "whirling" is known in the trade, but for the time being it has been taken into account only for the drill bit in order to reduce breakage and excessive wear of the cutting elements due to this dysfunctioning.